1. Field of the Invention
The present invention relates to a surface emitting laser and a surface emitting laser array. More particularly, the present invention relates to a vertical cavity surface emitting laser and a surface emitting laser array with a high-luminance optical output in fundamental transverse mode.
2. Description of the Related Art
The vertical cavity surface emitting laser (xe2x80x9cVCSELxe2x80x9d for short hereinafter) is expected to find use in a wide range of applications because of its many advantages over the edge emitting laser, such as lower production cost and higher yields and capability of being arranged easily in a two-dimensional array.
The structure, characteristics, and applications of VCSEL were mentioned in IEEE Journal of Quantum Electronics, 1988, 24, pp. 1845-1855, xe2x80x9cSurface Emitting Semiconductor Lasersxe2x80x9d by Kenichi Iga, Fumio Koyama, and Susumu Kinoshita. Since then, VCSEL has been greatly improved in characteristic properties and put to practical use in the area of optical communications.
However, the conventional VCSEL still has a small optical output in fundamental transverse mode (2-3 mW at the highest) and hence is limited in applications. If it has an increased optical output in fundamental transverse mode (say, 5 mW and above), then it will find use in the image-writing unit (such as laser beam printer) and magneto-optical disk unit.
One way to increase the optical out of VCSEL in fundamental transverse mode was described in IEEE Photonics Technology Letters, 4, pp. 374-377, 1993, xe2x80x9cTransverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasersxe2x80x9d by R. A. Morgan et al. According to this literature, the object is achieved if the emitting region for laser beam has an adequate opening.
This VCSEL has the general structure of proton implantation type, as shown in FIG. 16. It is composed of several layers formed on an n-type GaAs substrate (not shown). On the substrate is formed a lower n-type DBR (Distributed Bragg Reflector) layer 161 including layers of AlAs and Al0.16Ga0.84As deposited alternately 28.5 periods, having a carrier density of 3xc3x971018 cmxe2x88x923. On this lower DBR layer 161 is formed an undoped active layer region 162 including an active layer of quantum well structure and a spacer layer. On this active layer region 162 is formed an upper p-type DBR layer 163 including layers of AlAs and Al0.16Ga0.84As deposited alternately 20 periods, with Al0.58Ga0.42As placed at interface, having a carrier density of 3xc3x971018xcx9c2xc3x971019 cmxe2x88x923. On this upper p-type DBR layer 163 is formed a p-side electrode 164 with an opening 166 which defines the laser beam emitting region with a diameter W. The upper p-type DBR layer 163 is surrounded by a high-resistance region 165 formed by proton implantation which limits the region for current confinement into the active layer.
Incidentally, an n-side electrode (not shown) is formed on the underside of the substrate (not shown).
The VCSEL constructed as mentioned above is said to increase in the optical output in fundamental transverse mode if the size (or diameter g) of the current injection region and the opening W of the emitting region 166 are optimized. However, the optical output in fundamental transverse mode is still only 1.5 mW at the maximum. This output is too small for the laser to be used satisfactorily for the magneto-optical disk unit.
To address this problem, there has been proposed a VCSEL with a high-luminance optical output in fundamental transverse mode (Japanese Patent Laid-open H10-56233). According to this disclosure, the object of increasing the optical output in fundamental transverse mode is achieved by selectively controlling the lasing condition that permits the high-order transverse mode to occur secondarily in addition to the fundamental transverse mode. Because the fundamental transverse mode oscillation in VCSEL occurs at the center of the optical waveguide (or in the vicinity of the optical axis) and the high-order transverse mode oscillation occurs at a place away from the optical axis, it is possible to increase the optical output in fundamental transverse mode if the lasing condition is controlled such that the optical loss of the cavity gradually increases with the increasing distance from the optical axis and the injection current increases accordingly and multiple mode oscillation is suppressed.
To be more specific, the VCSEL is explained with reference to FIG. 17. It consists of a conductivity-type semiconductor substrate 171, a lower DBR layer 172, an upper DBR layer 174 whose conduction mode is opposite to that of the lower DBR layer 172, an active layer region 173 interposed between the lower DBR layer 172 and the upper DBR layer 174, a low reflectance zone 175 formed by ion implantation, a loss-determining element 176, and electrodes 177 and 178. It emits the laser beam along the optical axis 179.
The loss-determining element 176 has a concave shape so that the optical loss of the cavity gradually increases in going away from the optical axis 179 in the direction perpendicular to the optical axis 179. The concave loss-determining element 176 both diffracts the laser beam from the cavity and diffuses sideward (or defocuses) the laser beam from the cavity.
Therefore, this loss-determining element 176 causes the refraction loss to increase with the increasing distance from the optical axis 179 in the direction perpendicular to the optical axis 179, and the optical loss of the cavity increases accordingly. On the other hand, in VCSEL, the fundamental transverse mode oscillation occurs in the vicinity of the optical axis 179 and the high-order transverse mode oscillation occurs at a position away from the optical axis 179.
As the result, the optical loss of the cavity increases for the high-order transverse mode, the threshold current density necessary for the laser oscillation of high-order transverse mode to start increases, and the maximum fundamental transverse mode optical output greatly increases.
As mentioned above, the technology disclosed in Japanese Patent Laid-open H10-56233, in principle, makes it possible to increase the output in fundamental transverse mode. However, it also has the disadvantage of adversely affecting the fundamental transverse mode characteristics and presenting difficulties in forming stably the loss-determining element 176 of prescribed shape.
In other words, the technology disclosed in Japanese Patent Laid-open H10-56233 utilizes the fact that, in VCSEL, the fundamental transverse mode oscillation occurs at the center of the optical waveguide (in the vicinity of the optical axis) and the high-order transverse mode oscillation occurs at a position away from the optical axis, thereby causing the reflectivity of the cavity to gradually decrease in going from the center to the periphery. That is, it causes the optical loss to increase gradually and thereby suppresses the laser oscillation in high-order transverse mode.
On the other hand, VCSEL is usually has a small active region, as explained in xe2x80x9cSurface Emitting Laserxe2x80x9d by K. Iga and F. Koyama, issued by Ohm-sha, 1990. Therefore, it requires that the cavity have a high reflectance. In fact, the cavity for VCSEL under study today has a reflectance greater than 99%. Conversely, if the reflectance of the cavity is low, the threshold current density increases, making it difficult for laser oscillation to take place.
As matter of fact, the VCSEL disclosed in Japanese Patent Laid-open H10-56233 is constructed such that the reflectance of the cavity decreases at a position only slightly away from the optical axis 179. This suppresses not only the laser oscillation of the high-order transverse mode but also the laser oscillation of the fundamental transverse mode. As the result, this VCSEL does not provide a sufficiently high luminance fundamental transverse mode optical output.
In addition, the VCSEL disclosed in Japanese Patent Laid-open No. 56233/1998 is characterized by that the loss-determining element 176 has a curved surface (either concave or convex) as shown in FIG. 17. Thus the process for shaping the loss-determining element 176 is important, and it is detailed in Japanese Patent Laid-open H10-56233.
An example of the process is explained below. First, a photoresist 182 is applied to the surface of the layer 181 to be shaped convex, as shown in FIG. 18(a). The photoresist 182 is made into a cylindrical photoresist column 183, as shown in FIG. 18(b), by ordinary steps of exposure, development, and baking. The photoresist column 183 is heated at about 250-300xc2x0 C. for about 5-20 minutes, so that it deforms into a layer 184 with a convex surface, as shown in FIG. 18(c). The layer 184 retains the convex shape even after it has been cooled to room temperature.
Then, dry etching is carried out with a reactive ion etchant (RIE). During dry etching, the layer 184 functions as an etching mask, thereby causing the structure 185 with a convex surface to be formed, as shown in FIG. 18(d).
The above-mentioned process for forming the structure with a convex surface may be modified such that the photoresist column 183 is formed near the periphery in place of the center on the layer 181. In this way it is possible to form a structure having a concave surface at the center on the layer 181.
Although the layer 184 functioning as an etching mask should be formed with a prescribed curved surface at a prescribed position, it is considerably difficult to form the curved surface at any position with good reproducibility even with the present-day etching technology. This problem is serious particularly in the case where a large number of VCSEL elements are arranged in a two-dimensional array.
Moreover, forming the loss-determining element 176 with a specific curved shape and a specific thickness presents serious difficulties in terminating the etching at a proper position at the time of or after the time of disappearance of the layer 184 which has functioned as the etching mask in the RIE step.
In the case where a large number of VCSEL elements are arranged in a two-dimensional array, it is very difficult to accurately control the etching selectivity between the materials constituting the photoresist column 183 and the loss-determining element 176 on a single substrate or on two or more substrates. This in turn presents another serious difficulties in producing VCSEL elements which are uniform in the reflectance of the loss-determining element 176.
As mentioned above, it is very difficult to eliminate or minimize the variation in the shape and film thickness of loss-determining elements 176 among VCSEL elements on a single substrate or two or more than two substrates or among VCSEL elements of different lots.
On the other hand, the concave surface of the loss-determining element 176 causes the optical loss of the cavity to gradually increases with increasing distance from the optical axis 179, thereby increasing the injection current, preventing the shift to the laser oscillation of high-order transverse mode, and enabling the laser oscillation of fundamental transverse mode. Therefore, if the shape of the concave surface of the loss-determining element 176 differs, the optical output of VCSEL at which the shifting to the laser oscillation of high-order transverse mode occurs differs accordingly (or the maximum optical output of the fundamental transverse mode differs accordingly). As the result, the maximum optical output of the fundamental transverse mode varies among VCSEL elements on a single substrate or two or more than two substrates or among VCSEL elements of different lots. Thus it is industrially difficult to apply the technology disclosed in H10-56233 to applications which need the high-luminance optical output in fundamental transverse mode.
It is an object of the present invention to provide a surface emitting laser and a surface emitting laser array with a high luminance optical output in fundamental transverse mode almost uniform independently of positions, which can be produced easily with good reproducibility.
The present inventors carried out a series of researches which led to the finding that the above-mentioned problems can be solved by the following means.
The present invention covers a surface emitting laser of the type having an active layer region composed of an active layer and spacer layers deposited on both sides thereof and reflection layers deposited on both sides of the active layer region, which comprises a first mode control layer which is deposited at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and also at the position where the laser beam is received before its emission and which reflects the laser beam toward the reflection layer, and a second mode control layer (transparent) into which comes the laser beam reflected by the reflection layers and the first mode control layer.
One embodiment of the present invention is a surface emitting laser of the type having an active layer region composed of an active layer and spacer layers deposited on both sides thereof and reflection layers deposited on both sides of the active layer region, which comprises a first mode control layer which is deposited at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and also on the outside of at least one of the reflection layers and which reflects the laser beam toward at least one of the reflection layers, and a second mode control layer (transparent) which is deposited at least at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and is deposited between at least one of the reflection layers and the first mode control layer and which lowers the reflectance for the wavelength of laser oscillation at the periphery of the center of laser emission.
Another embodiment of the present invention is a surface emitting laser of the type having an active layer region composed of an active layer and spacer layers deposited on both sides thereof and reflection layers deposited on both sides of the active layer region, which comprises a first mode control layer which is deposited at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and also on the outside of at least one of the reflection layers and which reflects the laser beam toward at least one of the reflection layers, and a second mode control layer (transparent) which is deposited at least at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and is deposited between at least one of the reflection layers and the first mode control layer and which lowers the effective refractive index for the wavelength of laser oscillation at the periphery of the center of laser emission.
Another embodiment of the present invention is a surface emitting laser of the type having an active layer region composed of an active layer and spacer layers deposited on both sides thereof and reflection layers deposited on both sides of the active layer region, which comprises a first mode control layer which is deposited at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and also on the outside of at least one of the spacer layers and which reflects the laser beam toward at least one of the spacer layers, and a second mode control layer (transparent) which is deposited at least at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and is deposited between at least one of the reflection layers and the first mode control layer and which lowers the reflectance for the wavelength of laser oscillation at the periphery of the center of laser emission.
Another embodiment of the present invention is a surface emitting laser of the type having an active layer region composed of an active layer and spacer layers deposited on both sides thereof and reflection layers deposited on both sides of the active layer region, which comprises a first mode control layer which is deposited at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and also on the outside of at least one of the spacer layers and which reflects the laser beam toward at least one of the spacer layers, and a second mode control layer (transparent) which is deposited at least at the periphery of the center of emission of the laser beam which has been evolved in the active layer region and is deposited between at least one of the reflection layers and the first mode control layer and which lowers the effective refractive index for the wavelength of laser oscillation at the periphery of the center of laser emission.
According to the present invention, the surface emitting laser array is formed by arranging in an array a plurality of surface emitting layers of the present invention.
The surface emitting laser constructed as mentioned above selectively suppresses the laser oscillation in high-order transverse mode that secondarily occurs, thereby increasing the optical output in fundamental transverse mode, without impairing the characteristics of the laser oscillation in fundamental transverse mode. In addition, it is constructed such that the effective refractive index at the periphery of the emission center for the laser beam of the cavity is lower than that at the emission center for the laser beam of the cavity. This permits the refractive index waveguide to be formed in the laser cavity, thereby enabling the efficient laser oscillation in fundamental transverse mode. The above-mentioned structure permits the surface emitting laser with a high-luminance optical output in fundamental transverse mode to be produced economically in high yields by simple steps with good reproducibility.